features/shard: Fix vm corruption upon fix-layout

shard's writev implementation, as part of identifying
presence of participant shards that aren't in memory,
first sends an MKNOD on these shards, and upon EEXIST error,
looks up the shards before proceeding with the writes.

The VM corruption was caused when the following happened:
1. DHT had n subvolumes initially.
2. Upon add-brick + fix-layout, the layout of .shard changed
   although the existing shards under it were yet to be migrated
   to their new hashed subvolumes.
3. During this time, there were writes on the VM falling in regions
   of the file whose corresponding shards were already existing under
   .shard.
4. Sharding xl sent MKNOD on these shards, now creating them in their
   new hashed subvolumes although there already exist shard blocks for
   this region with valid data.
5. All subsequent writes were wound on these newly created copies.

The net outcome is that both copies of the shard didn't have the correct
data. This caused the affected VMs to be unbootable.

FIX:
For want of better alternatives in DHT, the fix changes shard fops to do
a LOOKUP before the MKNOD and upon EEXIST error, perform another lookup.

Change-Id: I8a2e97d91ba3275fbc7174a008c7234fa5295d36
BUG: 1440051
RCA'd-by: Raghavendra Gowdappa <rgowdapp@redhat.com>
Reported-by: Mahdi Adnan <mahdi.adnan@outlook.com>
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: https://review.gluster.org/17010
Smoke: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>

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